Apparatus for manufacturing chlorine dioxide

ABSTRACT

Chlorine dioxide is produced by electrolysis of a solution of chlorite and by using a compact apparatus which provides continuously a uniform concentration of chlorine dioxide, which can be operated easily, which discharges a spent electrolyte that can be treated easily and which achieves highly efficient use of chlorite and high current efficiency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a apparatus for electrolytic production ofchlorine dioxide used in bleaching of paper pulp and disinfection ordeodorization of potable water. More particularly, this inventionrelates to a apparatus for producing chlorine dioxide by electrolyzing asolution of alkali metal chlorite supplied to an anode compartment in adiaphragm electrolytic cell.

2. Description of Prior Art

Chlorine dioxide is produced by a chemical method wherein chlorate orchlorite is used as a starting material or by an electrochemical methodwherein chlorine dioxide is generated by electrolysis of chlorite. Themethod which obtains chlorine dioxide from chlorate uses a reducingagent that acts on the chlorate in a strong acid solution and the methodhas various modifications according to the type of reducing agent. Atypical example uses sodium chloride as the reducing agent (e.g. U.S.Pat. No. 3,563,702). Examples of the chemical method includedecomposition of chlorite with acid (e.g. Swiss Patent No. 527,126) andreaction between chlorite and chlorine (e.g. French Patent No.2,086,624). One example of the electrochemical method is diaphragmelectrolysis wherein a chlorite solution is used as anolyte and causticalkali or sodium chloride solution is used as catholyte (e.g. U.S. Pat.No. 3,763,006). In any of these methods, chlorine dioxide whoseconcentration is more than 15 vol% is explosive and hence is usuallyrecovered after being diluted with an inert gas such as air.

The chemical method, particularly the one that uses chlorate isprimarily used to produce chlorine dioxide for pulp bleaching and is inmost cases operated on a relatively large scale. Liquid chlorine isconventionally used to disinfect potable water, but is has a tendency toform trihalomethane (THM and primarily chloroform) being a suspectedcarcinogen in water, so researchers are getting interested in the use ofchlorine dioxide instead of liquid chlorine (Journal of the Society ofWaterworks, No. 546, Mar. 1980, pp. 123-124). For use in water treatmentplants, a small system for producing chlorine dioxide is a must. Forexample, a chemical system that is designed to generate less than about100 kg of chlorine dioxide per day involves a side reaction and withthis system, it is difficult to control the complex mechanism of thereaction of the formation of chlorine dioxide, and hence a lot ofequipment cost and labor is required to achieve the proper control ofchlorine dioxide produced. What is more, such system is not completelysafe. On the other hand, the electrochemical method is adaptive to asmall-scale apparatus, involves less side reaction and produces purechlorine dioxide, is capable of controlling the production of chlorinedioxide simply by controlling electrolytic current according to a changein the quality of water supplied to the water treatment plant or achange in the demand for water at different time during the day.Chlorine dioxide may explode at high concentrations, but unlike thechemical method the electrochemical method is very safe because when airblowing is suspended by power failure or some other reasons, theproduction of chlorine dioxide is also stopped.

The conventional electrochemical method for production of chlorinedioxide is performed either batchwise or continuously. In the batchwiseprocess, a chlorite solution in an anode compartment in the diaphragmelectrolytic cell is electrolyzed until the chlorite content becomessmall, and thereafter, the anolyte is replaced by a fresh supply forstarting electrolysis again. In the continuous process, a chloritesolution is continuously supplied to the anode compartment to maintainthe chlorite content constant throughout the electrolysis. One defect ofthe batchwise process is that the operation is suspended at every anodereplacement which is usually done every 1 to 3 days. To extend thereplacement interval, much anolyte is necessary, resulting in anincrease in equipment cost and installation area. In addition, the spentanolyte contains a considerable amount of chlorite and chlorine dioxidedissolved therein, and this means low efficiency in use of chlorite.Since a large amount of spent anolyte is discharged at one time, itstreatment is difficult and requires a lot of personnel expenses. Thecontinuous process provides a uniform concentration of chlorine dioxide,but much chlorine dioxide is dissolved away in the anolyte continuouslydischarged, and such discharged catholyte is difficult to treat, and theefficiency in use of chlorite is low. A continuous method is known thatis designed for increasing the efficiency in use of chlorite bysupplying a concentrated solution of chlorite prepared by adding achlorite crystal to the discharged anolyte. But even in this method,salts in the crystal and those produced during electrolysis graduallybuild up and anolyte replacement is unavoidable. What is more, it isdifficult to supply a constant amount of the crystal. Therefore, thereis a great demand for an electrolytic process for production of chlorinedioxide which provides continuously a uniform concentration of chlorinedioxide, can be carried out with a compact apparatus, can be operatedeasily, discharges a spent electrolyte that can be treated easily, andwhich achieves highly efficient use of chlorite and high currentefficiency.

SUMMARY OF THE INVENTION

It is an object of the invention, therefore, to provide a process forproducing chlorine dioxide by electrolysis of a solution of chloritewhich provides continuously a uniform concentration of chlorine dioxide,which can be operated easily, which discharges a spent electrolyte thatcan be treated easily and which achieves highly efficient use ofchlorite and high current efficiency.

It is another object of this invention to provide a compact apparatusfor production of chlorine dioxide by electrolysis of a solution ofchlorite which provides continuously a uniform concentration of chlorinedioxide, which can be operated easily, which discharges a spentelectrolyte that can be treated easily and which achieves highlyefficient use of chlorite and high current efficiency.

Other objects and advantages of the present invention may becomeapparent to those skilled in the art from the following description anddisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow sheet of the process of this inventionaccording to one embodiment.

FIg. 2 is a schematic longitudinal sectional view of one embodiment ofan apparatus except for an electrolytic cell according to thisinvention.

FIGS. 3A, 3B and 3C are schematic cross section views taking along lineA-A', B-B' and C-C' in FIG. 2, respectively.

FIGS. 4A, 4B and 4C are a front view, a left side view and a right sideview of an electrolytic cell of a filter-press assembly of bipolardesign having 5 unit cells according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention provides a process for generating chlorine dioxide byelectrolysis of a solution of chlorite, wherein a solution of alkalimetal chlorite is supplied to a supply tank, an overflow from saidsupply tank is combined with an anolyte circulating between an anodecompartment in a diaphragm electrolytic cell and a stripping tank (thepH of said anolyte has been adjusted to a suitable value with part ofthe circulating catholyte) before the latter enters the anodecompartment, the combined fluid is sent to the anode compartment whereit is electrolyzed and from which it is returned to the stripping tank,an anolyte overflow from said stripping tank is discharged from thesystem through an auxiliary stripping tank, air directed into theauxiliary stripping tank is then supplied to the stripping tank to stripthe anolyte of the chlorine dioxide generated in the anode compartment,said air containing chlorine dioxide is advanced into the supply tankand discharged from the system, the catholyte being circulated between acathode compartment in the electrolytic cell and a circulation tank,diluting water being added to said circulation tank, part of thecatholyte being discharged from the system, another part of thecatholyte being combined with said circulating anolyte at aneutralization point upstream of the point where it is combined with theoverflowed chlorite solution from a supply tank. The process of thisinvention is particularly suitable for continuous operation, but it maybe operated batchwise for a certain concentration of chlorite.

One embodiment of the process of this invention is hereunder describedby reference to the accompanying FIGS. 1 to 4. In the drawings, adiaphragm electrolytic cell 1 may be of bipolar or unipolar design, andthe following description assumes that the cell is of bipolar designcomprising a terminal anode 2, a terminal cathode 3, and an intermediateelectrode 4, and the cell is divided into anode compartments 6, 6' andcathode compartments 7, 7' by diaphragms 5, 5' placed between eachelectrode. The anode is made of an electrically conductive andcorrosion-resistant substrate such as titanium which is coated on theanode compartment side with a common anode catalytic active materialsuch as a metal of platinum group, an alloy thereof, an oxide thereof,magnetite or ferrite. The diaphragm may be made of ceramic, asbestos orcation exchange membrane, and preferably it is made of a fluoro-carboncation exchange membrane having high perm selectively and durability. Toachieve a compact configuration, the electrolytic cell is perferably ofbipolar, filter press type.

The solution of alkali metal chlorite is continuously supplied through apipe 8 which extends close to the bottom of a supply tank 9. Examples ofthe alkali metal are sodium, potassium and lithium, and the sodium iscommonly used. The chlorite solution from the tank 9 flows down anoverflow pipe 10 and is combined at the junction 28 with the circulatoryanolyte, which has been adjusted to a predetermined pH through mixingwith part of the circulating catholyte at a junction 13 on an anolytesupply pipe 12 connected between a stripping tank 11 and the electrlyticcell 1. The mixture obtained at the junction 28 is then directed to theanode compartment 6, 6'. The electrolyzed chlorite solution is returnedto the stripping tank 11 through an anolyte return pipe 14. Part of theanolyte flows down an overflow pipe 15 which extends close to the bottomof an auxiliary stripping tank 16 from which it is discharged to theoutside of the system through a pipe 17.

Air is introduced through a pipe 18 which extends close to the bottom ofan auxiliary stripping tank 16 to strip the anolyte of the dissolvedchlorine dioxide through gas-liquid contact, and the mixture of air andchlorine dioxide then is directed to the stripping tank 11 through apipe 19. In the stripping tank 11, the anolyte is stripped of thedissolved chlorine dioxide to a given gas-liquid equilibriumconcentration. The chlorine dioxide diluted with air is then directed tothe supply tank 9 through a pipe 20, and from that tank, it is recoveredthrough a discharge pipe 21. In FIG. 2, numerals 29, 30 and 31 aredistributors (perforated plates) through which introduced air isuniformly dispersed.

The catholyte in a circulation tank 22 is directed to the cathodecompartments 7, 7' of the electrolytic cell 1 through a pipe 23, andafter being electrolyzed in that cell, the catholyte is returned to thecirculation tank 22 through a return pipe 24, thus circulating betweenthe circulation tank 22 and the cathode compartments 7, 7'. To inhibitthe increase in the alkali concentration of the catholyte, dilutingwater is kept supplied to the circulation tank 22 through a pipe 25during electrolysis, and part of the catholyte is discharged from thesystem through a pipe 26, and another part is branched from thecatholyte return pipe 24 and sent through an alkali supply pipe 27 whichis connected to the anolyte supply pipe 12 at a junction 13 upstream thejunction 28, and at that junction 13, the circulating anolyte isneutralized with the alkali in the catholyte. Alternatively, the alkalimay be neutralized with hydrochloric acid supplied to the circulationtank 22 through the pipe 25.

In an especially preferred embodiment (see FIGS. 2 and 3) the apparatusis arranged in a single cylindrical reactor (except for the electrolysiscell). The circulation tank 22 for the catholyte and the auxiliarystripping tank 16 are located next to each other on the lower floor ofthe reactor. The stripping tank 11 is arranged above. The supply tank 9forms the upper floor of the reactor. The tanks which are arranged oneabove the other are connected with each other via gas and liquid pipesas described before. These pipes are located substantially in theinterior of the reactor. The air which is blown in for stripping theanolyte rises from the auxiliary stripping tank into the supply tank 9via the stripping tank 11. The mixture of air and ClO₂ flows firstupwards (up to above the level of the liquid in the stripping tank 11 ofthe supply tank 9) through the pipes 19 or 20 in the manner to be seenfrom FIG. 2 and then downwards below the perforated plate 30 and 29,respectively, through a concentric exterior pipe.

When a D.C. current from a D.C. source (not shown) is applied to theelectrolytic cell 1, an electrode reaction represented by the formula(1) takes place in the anode compartments 6, 6' to generate chlorinedioxide. Alkali and hydrogen are evolved in the cathode compartments 7,7' as in the conventional electrolysis of sodium chloride.

    ClO.sub.2.sup.- →ClO.sub.2 +e                       (1)

The chlorine dioxide evolved through the reaction of (1) is highlysoluble and dissolves in the circulating anolyte, and air blown into thestripping tank 11 reduces the chlorine dioxide content in the anolyteuntil a gas-liquid equilibrium in the stripping tank is obtained, andthe displaced chlorine dioxide is recovered in the form of a mixturewith air. Since only a low voltage is required for decomposing thechlorite ion, only a little electrical power is required to producechlorine dioxide. Even if sodium chloride and other salts are present inthe reaction system, no chlorine is evolved, because the voltagerequired for decomposing the chlorite ion is lower than that requiredfor decomposing chloride ion. If a high concentration of chlorite ion issupplied to the electrolytic cell 1, the pH of the anolyte is usuallyheld close to neutral, but when the concentration of chlorite ion isless than about 40 g/l, the cell voltage increases slightly and bothchlorite ion and chloride ion are electrically discharged to evolvechlorine as well as chlorine dioxide. The evolved chlorine is soonhydrolyzed or dissolved in the anolyte, and the pH of the anolyte isgradually decreased due to the hydrochloric acid produced. When the pHof the anolyte is reduced to about 2, chlorine dioxide is evolved andthe presence of chlorite ion is substantially zero, and hence chlorinegas is evolved. Accordingly, to achieve efficient use of chlorite and todischarge spent anolyte that is substantially free of chlorite ion, thepH of the anolyte at the outlets of the anode compartments 6, 6' must belower than a certain level which is 2 for the purposes of thisinvention. To meet this end, the amount of anolyte circulating betweenthe anode compartments 6, 6' and the stripping tank 11 is controlled.Usually, the flow rate and concentration of the chlorite solution aredetermined by a theoretical consumption amount corresponding to theelectrochemical equivalent amount of chlorite ion (2.518 g per amperehour). If it is assumed that the so determined amount of chlorite ion isdiluted with the circulating anolyte before it enters the anodecompartments 6, 6' where almost all of the ion is decomposed to chlorinedioxide and electron, the flow rate of the circulating anolyte isdetermined by the concentration of chlorite ion in the mixture ofchlorite solution and anolyte. This means the flow rate of thecirculating anolyte is the ratio of the amount of chlorite ion supplied,to the predetermined concentration of chlorite ion in the mixture ofchlorite solution and anolyte. In this invention, the concentration ofchlorite ion in the mixture of chlorite solution and anolyte is limitedto a range of from 10 to 40 g/l. If the concentration is lower than 10g/l, the current efficiency drops significantly while the cell voltageincreases greatly to make economical electrolysis impossible. If theconcentration is higher than 40 g/l, no drop in the pH of the anolyteoccurs, and since less anolyte is circulated, not all of the chlorinedioxide produced can be dissolved in the circulating anolyte flowingthrough the return pipe 14, and in consequence, part of the chlorinedioxide assumes a gaseous form which is undesirable for itsexplosiveness.

If the pH of the anolyte at the outlets of the anode compartments 6, 6'is adjusted to lower than 2, not only chlorine dioxide but also chlorinegas is evolved, but the gas mixture from the stripping tank 11 isdirected to the chlorite solution in the supply tank 9, and chlorinereacts with the chlorite ion to form chlorine dioxide by taking thefollowing reaction course:

    2ClO.sub.2.sup.- +Cl.sub.2 →2ClO.sub.2 +2Cl.sup.-   (2)

Therefore, the gas discharged through the pipe 21 comprises only air andchlorine dioxide and does not contain chlorine. The chlorine dioxide andchlorine dissolved in the anolyte which has overflowed into theauxiliary stripping tank 16 is removed by an additional supply of freshair, and the effluent discharged through the pipe 17 contains nochlorine dioxide, and there occurs little loss of chlorine dioxideproduced by electrolysis. In the reaction represented by the formula(2), the chlorite ion supplied is consumed by chlorine gas, but sinceone mol of chlorine yields 2 mols of chlorine dioxide, there is noelectrochemical stoichiometric loss.

The anolyte whose pH has been adjusted to less than 2 is then suppliedfrom the stripping tank 11 to the anode compartments 6, 6', but beforeit dilutes the chlorite solution, it is neutralized with the alkaliproduced as a by-product in the cathode compartments 7, 7'. By thisneutralization, the pH of the analyte is adjusted to a level between 3and 8. If the pH is less than 3, the chlorine evolved is hydrolyzed tohydrochloric acid, and if it is mixed with the chlorite ion, said ion iseasily activated and decomposed as represented by the following formula(3) before it is electrolyzed. Since this reaction produces only 4 molsof chlorine dioxide from 5 mols of chlorite ion, an electrochemicalstoichiometric loss occures in the reaction.

    5ClO.sub.2.sup.- +4HCl→4ClO.sub.2 +5Cl.sup.- +2H.sub.2 O (3)

If the pH is higher than 8, the dissolved chlorine dioxide easilyreturns to chlorite ion. Preferably, the pH of the anolyte isneutralized to a level in the range of from 4 to 7. The hypochlorite ionproduced by hydrolysis of chlorine also easily reacts with the chloriteion at an anolyte pH of 3 to 8, following the course represented below,but no electrochemical stoichiometric loss occurs in this reaction.

    2ClO.sub.2.sup.- +ClO.sup.- +H.sub.2 O→2ClO.sub.2 +Cl.sup.- +2OH.sup.-                                                (4)

The amount of chlorine dioxide is substantially proportional toelectrolytic current and a constant concentration of chlorine dioxide iskept produced, and the concentration of chlorine dioxide produced iscontrolled by the amount of air supplied to the system. For the purposesof this invention, the concentration of chlorine dioxide is limited to alevel between 3 and 15 vol%. If the concentration is lower than 3%, thehuge amount of air to be supplied reduces the operation efficiency, andif the concentration is higher than 15%, the possibility of explosion isincreased. Preferably, the concentration of chlorine dioxide is held ina range between 5 and 12%. During electrolysis, the bath temperature ofthe stripping tank 11 is held at between 10° and 90° C. A temperaturelower than 10° C. is not economical because of increased cell voltage,and a temperature higher than 90° C. accelerates corrosion or otherattacking of the equipment, while also results in inefficient operation.A diaphragm made of cation exchange membrane is preferred to a ceramicdiaphragm because it minimizes the migration of hydroxyl ion into theanode compartment and hence inhibits the formation of a by-productchlorate in the anode compartment, with the result that electrolysis isperformed at high current efficiency.

Chlorine dioxide is unstable and may cause explosion at highconcentration, so the system is preferably operated under vacuum byinstalling a suction pump or ejector downstream of the pipe 21. Toprotect the diaphragm or to prevent the migration of an interfering ionthrough the diaphragm, the circulation tank 22 must also be placed invacuum so that the pressures in the anode and cathode compartmentsseparated by the diaphragm are almost equal. It is also necessary tocirculate the catholyte at a flow rate substantially equal to that ofthe anolyte. To prevent the migration of hydroxyl ion through thediaphragm, the pressure in the anode compartment is usually higher thanthat in the cathode compartment by not more than 100 mmH₂ O, preferablynot more than 50 mmH₂ O. Another reason to place the circulation tank 22in vacuum is to dilute the evolved hydrogen gas until its concentrationis less than the lower limit for explosion.

The start and stop of the system can be controlled by loading anautomatic arithmetic unit with the electrolytic current to be applied tothe electrolytic cell and the amount of supply of the chlorite solution.What is more, optimum control of the amount and concentration ofchlorine dioxide to be produced can be achieved by modifying theelectrolytic current and the amount of air supply.

According to the method of this invention, electrolysis can beaccomplished while the chlorite solution is supplied to the anodecompartment of the diaphragm electrolytic cell by way of the supplytank. In addition, by letting air pass through the auxiliary strippingtank, stripping tank and supply tank successively, chlorine-freechlorine dioxide of high purity and constant concentration can berecovered continuously without interrupting the operation. Theconcentration of chlorine dioxide being recovered can be readilymodified by controlling the electrolytic current. Since chlorine dioxidegas that contains chlorine is purified in the supply tank, highlyefficient decomposition of the chlorite is also possible. Almost all ofthe residual chlorine dioxide is recovered in the auxiliary strippingtank and the efficiency of use of the chlorite is almost 100%.Therefore, the waste anolyte continuously coming out of the system canbe discharged into watercourses without requiring any complex treatmentother than neutralization. The continuous electrolytic process of thisinvention achieves a current efficiency of as high as 88 to about 90%.This way, the process of this invention is so simple to operate that itcan be automated to reduce labor requirements greatly.

The following examples are given to further illustrate this invention,but it should be understood that the invention is by no means limitedthereto. On the contrary, they are given only to clarify some of theessential embodiments of the present invention.

EXAMPLE 1

A system for continuous production of chlorine dioxide by electrolysiswas installed as represented in FIG. 1. The supply tank, stripping tank,auxiliary stripping tank and catholyte circulating tank were made ofrigid vinyl chloride cylinders that measured 200 mm (I.D.)×450 mm(height), 250×600 mm, 200×500 mm, and 200×500 mm; the air inlet pipe,gas pipes and all other pipes for gas stream were made of 13A rigidvinyl chloride; and the overflow pipes, anolyte supply pipe and allother pipes for liquid stream were made of 16A rigid vinyl chloride. Theelectrolytic cell used a Nafion diaphragm, a Du Pont sulfonatedfluorocarbon cation exchange membrane, and the cell was a filter-pressassembly of bipolar design having 5 unit cells each comprising an anodecompartment separated from a cathode compartment by the diaphragm. Theassembled cell had the outer dimensions of 550 mm (height)×350 mm(width)×250 mm (thickness). Each anode comprised a 3 mm thick titaniumplate having a platinum coating on the anode compartment side. Asanolyte, a sodium chlorite solution was supplied that contained 225 g/lof chlorite ion and 20-25 g/l of sodium chloride. As catholyte, anaqueous solution of caustic soda was supplied at 25 g/l only at thestart of the operation. The anolyte and catholyte were circulated byrespective magnet pumps, and air was forced into the auxiliary strippingtank with a Nash pump. The system was operated for a continuous periodof about 3 weeks an electrolytic current of 100 amperes, with the sodiumchlorite solution being supplied to the supply tank at a rate of 93ml/min and air supplied at 67 liters/min (N.T.P.). To hold theconcentration of chlorite ion after dilution with the circulatinganolyte at 20 g/l, the anolyte was circulated at a rate of about 1.0liter/min. The pH of the circulating anolyte before it was used todilute the chlorite ion was adjusted to about 5 with the alkaliby-product, and the bath temperature of the stripping tank was held atabout 40° C.

During the electrolysis, the pH of the anolyte at the outlets of theanode compartments was about 1.3, and chlorine dioxide having a purityof 100 mol% was produced continuously in an amount of about 1.12 kg-ClO₂/hr. Its concentration remained substantially constant, i.e. ca.10.1vol%. The average cell voltage and current efficiency duringelectrolysis were about 5.3 volts and 88.9%, respectively. The amount ofchlorine detected in the product chlorine dioxide was substantiallyzero. The waste anolyte contained about 0.3 g/l of NaClO₂, 30 to 40 g/lof NaCl, and 10 to 20 g/l of NaClO₃, and the efficiency of use of thechlorite ion was 99.9%. Diluting water was supplied at 450 ml/min, andthe alkali concentration of the catholyte in the circulation tankremained substantially constant at 24.6-25.5 g/l.

EXAMPLES 2 TO 4

Electrolysis of sodium chlorite was conducted under the same operatingconditions as used in Example 1 except for the concentration of chloriteion after dilution, the flow rate of the circulating anolyte, the pH ofthe anolyte after neutralization, and the bath temperature of thestripping tank. The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________            circu-                                     conc. of                      ClO.sub.2.sup.-                                                                    lating        pH at the                                                                           conc.                                                                             amount of current                                                                           efficiency                                                                         Cl.sup.- in                   after                                                                              flow pH after                                                                           bath                                                                              outlet of                                                                           of  product                                                                             cell                                                                              effi-                                                                             of use of                                                                          product                       dilution                                                                           rate neutral-                                                                           temp.                                                                             anode com-                                                                          ClO.sub.2                                                                         ClO.sub.2                                                                           voltage                                                                           ciency                                                                            ClO.sub.2.sup.-                                                                    ClO.sub.2                  Ex.                                                                              (g/l)                                                                              (1/min)                                                                            ization                                                                            (°C.)                                                                      partment                                                                            (V %)                                                                             (kg/h)                                                                              (V) (%) (%)  (V %)                      __________________________________________________________________________    2  35   0.6  5.0  40  1.8   10.2                                                                              1.13  5.1 89.8                                                                              99.5 0                          3  20   1.0  7.0  40  1.5   10.0                                                                              1.11  5.3 88.2                                                                              99.8 0                          4  20   1.0  5.0  70  1.1   10.2                                                                              1.13  4.7 89.8                                                                              99.9 0                          __________________________________________________________________________

COMPARATIVE EXAMPLE 1

Electrolysis of sodium chlorite was conducted under the same operatingconditions as used in Example 1 except that no supply tank was used. Theresults are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________             circu-                                     conc. of                      ClO.sub.2.sup.-                                                                    lating        pH at the                                                                           conc.                                                                             amout of  current                                                                           efficiency                                                                         Cl.sup.- in                   after                                                                              flow pH after                                                                           bath                                                                              outlet of                                                                           of  product                                                                             cell                                                                              effi-                                                                             of use of                                                                          product                   Comp.                                                                             dilution                                                                           rate neutral-                                                                           temp.                                                                             anode com-                                                                          ClO.sub.2                                                                         ClO.sub.2                                                                           voltage                                                                           ciency                                                                            ClO.sub.2.sup.-                                                                    ClO.sub.2                 Ex. (g/l)                                                                              (1/min)                                                                            ization                                                                            (°C.)                                                                      partment                                                                            (V %)                                                                             (kg/h)                                                                              (V) (%) (%)  (V %)                     __________________________________________________________________________    1   20   1.0  5.0  40  1.3   9.0 1.01  5.2 80.2                                                                              99.9 0.5                       __________________________________________________________________________

COMPARATIVE EXAMPLES 2 TO 5

Electrolysis of sodium chlorite was conducted under the same operatingconditions as used in Example 1 except for the concentration of chloriteion after dilution, the flow rate of the circulating anolyte, and the pHof the anolyte after neutralization. The results are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________             circu-                            ClO.sub.2.sup.-                                                                        conc. of                      ClO.sub.2.sup.-                                                                    lating    pH at the                                                                           conc.                                                                             amount of current                                                                           in  efficiency                                                                         Cl.sup.- in                   after                                                                              flow pH after                                                                           outlet of                                                                           of  product                                                                             cell                                                                              effi-                                                                             waste                                                                             of use of                                                                          product                   Comp.                                                                             dilution                                                                           rate neutral-                                                                           anode com-                                                                          ClO.sub.2                                                                         ClO.sub.2                                                                           voltage                                                                           ciency                                                                            anolyte                                                                           ClO.sub.2.sup.-                                                                    ClO.sub.2                 Ex. (g/l)                                                                              (1/min)                                                                            ization                                                                            partment                                                                            (V %)                                                                             (kg/h)                                                                              (V) (%) (g/l)                                                                             (%)  (V %)                     __________________________________________________________________________    2    5   4.0  5.0  1.2   9.4 1.04  5.9 82.6                                                                              0.2 99.9 0                          3*  50* 0.4  5.0  --    --  --    --  --  --  --   --                        4   20   1.0  1.1  0.9   8.5 0.94  6.2 74.7                                                                              0.2 99.9 0                                       (no neu-                                                                      traliza-                                                                      tion)                                                           5   20   1.0  9.0  2.2   9.0 1.00  5.2 79.7                                                                              5.5 98.5 0                         __________________________________________________________________________     *Not all chlorine dioxide produced that flowed through the anolyte return     pipe could be dissolved in the circulating anolyte any longer, and part o     it was present in a gaseous form. So, the operation was suspended because     of increased possibility of explosion.                                   

What is claimed is:
 1. An apparatus for producing chlorine dioxide bythe electrolysis of a chlorite solution which comprises:an electrolyticcell for producing chlorine dioxide by electrolysis of a solution ofchlorite, said electrolytic cell including an anode compartment and acathode compartment, said electrolytic cell including means fordischarging anolyte from said anode compartment and means fordischarging catholyte from said cathode compartment; a supply tankhaving a first input for receiving an aqueous fresh chlorite solution, afirst output for supplying a chlorite containing solution, a secondinput for receiving a mixture of chlorine dioxide and air and a secondoutput for removing a mixture of chlorine dioxide and air from thesystem; a stripping tank having a first output for supplying an anolyteto the anode compartment of said electrolytic cell, a first inputcoupled to the anode compartment of said electrolytic cell for receivinganolyte containing chlorine dioxide dissolved therein, a second inputfor receiving a mixture of chlorine dioxide and air, said stripping tankstripping some of said anolyte of chlorine dioxide by said air containedin said mixture supplied thereto, a second output for supplying anolytecontaining dissolved chlorine dioxide to an auxiliary stripping tank,and a third output for supplying a mixture of chlorine dioxide and airto said supply tank; said auxiliary stripping tank including a firstinput for receiving anolyte from said stripping tank, a second input forreceiving fresh air, a first output for supplying a mixture of chlorinedioxide and air to said stripping tank, said auxiliary stripping tankstripping anolyte of chlorine dioxide dissolved therein by means of saidfresh air supplied thereto, and a second output for discharging a partof the anolyte contained in said auxiliary stripping tank from thesystem; a circulation tank coupled to the cathode compartment of saidelectrolytic cell and including a first input for receiving a firstportion of said catholyte discharged from said cathode compartment, asecond input for receiving a diluent to dilute the catholyte and tothereby control the alkali concentration of the catholyte in saidcirculation tank, a first output for supplying diluted catholyte to thecathode compartment of said electrolytic cell, and a second output fordischarging a part of the catholyte contained in said circulation tankfrom the system; means for supplying a second portion of the catholytedischarged from said cathode compartment to said first output of saidstripping tank; means for combining said second portion of saidcatholyte and said anolyte from said first output of said stripping tankto form a first mixture; and means for combining said first mixture withsaid chlorite containing solution provided at said first output of saidsupply tank to thereby form a second mixture, and means for supplyingsaid second mixture to said anode compartment of said electrolytic cell.2. The apparatus of claim 1, wherein said electrolytic cell comprises adiaphragm electrolytic cell.
 3. The apparatus of claim 2, wherein saiddiaphragm comprises a fluorocarbon cation exchange membrane.
 4. Theapparatus of claim 2, wherein said electrolytic cell comprises a bipolardiaphragm electrolyte cell.
 5. The apparatus of claim 2, wherein saidelectrolytic cell comprises a unipolar diaphragm electrolytic cell. 6.The apparatus of claim 1 comprises a source of diluent coupled to saidcirculation tank for supplying said diluent consisting essentially ofwater.
 7. The apparatus of claim 1 comprises a source of diluent coupledto said circulation tank for supplying said diluent consistingessentially of hydrochloric acid.
 8. The apparatus of claim 1, furthercomprising means for maintaining the anolyte of said stripping tank at atemperature of between 10° and 90° C.
 9. The apparatus of claim 1,further comprising means for maintaining the anode and cathodecompartments of said electrolytic cell at substantially the samepressure.
 10. The apparatus of claim 1, further comprising a vacuumproducing means connected to the second output of said supply tank. 11.An apparatus for producing chlorine dioxide by the electrolysis of achlorine solution which comprises:a substantially enclosed substantiallycylindrical reaction vessel having a bottom compartment, a middlecompartment and a top compartment, and an electrolytic cell locatedoutside of said cylindrical reaction vessel for producing chlorinedioxide by electrolysis of a solution of chlorite, said electrolyticcell including an anode compartment, a cathode compartment, means fordischarging anolyte from said anode compartment and means fordischarging catholyte from said cathode compartment; said uppercompartment comprising a supply tank having a first input for receivingan aqueous fresh chlorite solution, a first output for supplying achlorite containing solution, a second input for receiving a mixture ofchlorine dioxide and air and a second output for removing a mixture ofchlorine dioxde and air from said reaction vessel; said middlecompartment comprising a stripping tank having a first output forsupplying an anolyte to the anode compartment of said electrolytic cell,a first input coupled to the anode compartment of said electrolytic cellfor receiving anolyte containing chlorine dioxide dissolved therein, asecond input for receiving a mixture of chlorine dioxide and air, saidstripping tank stripping some of said anolyte of chlorine dioxide bysaid air contained in said mixture supplied thereto, a second output forsupplying anolyte containing dissolved chlorine dioxide to an auxiliarystripping tank, and a third output for supplying a mixture of chlorinedioxide and air to said supply tank; said bottom compartment comprisingsaid auxiliary stripping tank including a first input for receivinganolyte from said stripping tank, a second input for receiving freshair, a first output for supplying a mixture of chlorine dioxide and airto said stripping tank, said auxiliary stripping tank stripping anolyteof chlorine dioxide dissolved therein by means of said fresh airsupplied thereto, and a second output for discharging a part of theanolyte contained in said auxiliary stripping tank from said cylindricalreaction vessel; and a circulation tank coupled to the cathodecompartment of said electrolytic cell and including a first input forreceiving a first portion of said catholyte discharged from said cathodecompartment, a second input for receiving a diluent to dilute thecatholyte and to thereby control the alkali concentration of thecatholyte in said circulation tank, a first output for supplying dilutedcatholyte to the cathode compartment of said electrolytic cell, and asecond output for discharging a part of the catholyte contained in saidcirculation tank from said reaction vessel; means for supplying a secondportion of the catholyte discharged from said cathode compartment tosaid first output of said stripping tank; means for combining saidsecond portion of said catholyte and said anolyte from said first outputof said stripping tank to form a first mixture; and means for combiningsaid first mixture with said chlorite containing solution provided atsaid first output of said supply tank to thereby form a second mixture,and means for supplying said second mixture to said anode compartment ofsaid electrolytic cell.
 12. The apparatus of claim 11 furthercomprising:(a) means for dispersing the mixture of chlorine dioxide andair, which enters the supply tank through said second input, throughoutthe chlorite containing solution in said supply tank; (b) means fordispersing the mixture of chlorine dioxide and air which enters thestripping tank through said second input throughout the anolyte in saidstripping tank; and (c) means for dispersing fresh air which enters theauxiliary stripping tank throughout the anolyte in said auxiliarystripping tank.
 13. The apparatus of claim 12, wherein each of saiddispersing means comprises a plate having an upper and a lower majorsurface, said plate having uniformly distributed perforations connectingsaid upper and lower major surfaces, said plate having substantially thesame size and shape as the bottom surface of said tank and beingsubstantially parallel to the bottom surface of said tank, wherein saidmixture of chlorine dioxide and air or said fresh air is released intosaid tank below the lower major surface of said plate.
 14. The apparatusof claim 11, wherein said circulation tank is in said bottom compartmentof said reaction vessel.
 15. The apparatus of claim 11 comprises asource of diluent coupled to said circulation tank for supplying saiddiluent consisting essentially of water.
 16. The apparatus of claim 11comprises a source of diluent coupled to said circulation tank forsupplying said diluent consisting essentially of hydrochloric acid.